1. Field of the Invention
This invention relates to welds and methods and apparatus for detecting development of a failure within a weld, particularly useful in nuclear reactor systems.
2. Description of the Prior Art
In order to ensure reliability of any structural system including fastened components, such as those joined by welds, it is common practice for the welded area to be periodically inspected for evidence of crack initiation, metal separation or other failure. While many devices are available for such inspections, some provide indications not consistent with the degree of accuracy required, and some are not readily adaptable to remote use.
An application where accurate and rapid indications of weld failure are particularly desirable is in nuclear reactors, particularly in relation to main structural components within the reactor vessel which support the nuclear core. For example, while the core of fuel assemblies is typically bottom supported by a lower support structure, control rods which are reciprocatingly insertable to control core reactivity are top mounted. In the unlikely event of failure of the lower support structure attachment welds, undesirable separation of, or interference between, the control rods and fuel assemblies could result. In-service inspection programs have been instituted by the nuclear industry for inspection of such welds, including remote visual indicators useful, for example, in water-cooled reactors. With liquid metal cooled reactors, however, such visual indicators are not readily adaptable as a result of the opaque nature of the coolant. And, typical inspection systems require costly items such as multiple access ports and equipment and time consuming inspections carried out closely adjacent to the welded area.
In order to detect flaws in other reactor components, such as fuel rods, liquid metal reactors have been proposed which include the incorporation of tag gases within the fuel rods. The reactors include flow paths for directing tag gases released upon failure of a fuel rod to a monitoring and detection system which alerts the plant operator to such failure. Such reactors additionally are designed to prevent the accumulation of gas bubbles which could be swept through the core region. Accordingly, reactor internals are typically configured to vent any gas which enters the reactor system to a cover gas region above the core. The venting system thus permits gases released outside of the core to flow upwardly and eventually mix with any tag and fission product gases released from fuel assemblies within the core.
It is desirable to provide arrangements which allow rapid and accurate weld failure indication. It is particularly desirable to provide such arrangements in nuclear reactors, and to take advantage of existing system designs and configurations.